Yes it 100% slavic. The fact that to you Prelja it sound like mixture of Polish, Czech and Sloval and to me like mixture of Slovenian, Serbo-croatian and Polish proves that they are closest to Poles and that Slovene and Serbo-croatian has some kind of west slavic influence. I think West-south slavic people have some kind of west slavic origin.
I have been wondering about this for quite some time. Just to think; similarity of name for Croatian town Osijek and name Osiek for many Polish places isnt just accidence.